This invention refers to a kind of gamma radiation detector probe with a halogen quenched Geiger-Muller tube and to a procedure which extends the measuring range of the said tube, compensates for pulse losses due to dead time and avoids the saturation effects of the tube by simultaneously employing the electrical pulses and the current delivered by the Geiger tube when the latter is exposed to a gamma radiation field. Moreover, the procedure includes means of adjusting the response of the probe for a certain value of the exposure rate, regardless of the Geiger tube used, within those of the same type. Also includdedare means which prevent the temperature of the Geiger tube from dropping from a certain adjustable value, with the purpose of lengthening the life of the tube.
It is known that when the exposure rate of the gamma radiation which impinges on a Geiger-Muller (GM) tube is increased, the pulse losses due to dead time of the said tube become more significant, which causes a loss of proportionality between the exposure rate and the number of pulses delivered by the Geiger tube; therefore the use of the tube is limited unless corrections are made, with calculations that are more or less feasible and in all cases troublesome.
It is also known that by increasing the gamma radiation exposure rate to values higher than those that correspond to its measuring range, the electrical pulses which the tube delivers decrease in amplitude to the point of disappearing, due to a reduction of the electric charge per pulse as a result of the increase of the mean current which passes through the tube. This reduction of the amplitude and the disappearance of the pulses may produce dangerous situations and cause accidents due to false indications of the measuring equipment at high exposure rates.
The two preceding phenomena produce, as a result, a dependence of the current from the Geiger tube proportional to the logarithm of the exposure rate, until the constant value of the saturation current is reached.
Procedures have been employed for extending the range of use of the Geiger tubes and linearizing their response in relation to the exposure rate by pulsating the polarization high voltage of the tube or superimposing a voltage pulse upon the latter. This procedure has the disadvantage of having to produce electrical pulses of some hundreds of volts in amplitude.
In this invention simultaneous use is made of the voltage pulses generated by the Geiger tube and of the current which passes through it, with the net result of a linearization of the count rate-exposure rate characteristic of the probe, so that the latter delivers electrical pulses whose frequency has an appreciably linear relation to the exposure rate in the whole measuring range. The said pulse frequency can be measured with any frequency meter or suitable measuring equipment.
The probe has two channels for transmission of the signals which the Geiger tube produces when it is situated in a gamma radiation field.
The first channel is the pulse channel and it transmits a fixed fraction of those which the Geiger tube generates. Geiger tubes of the same type present approximately equal count rates for a given exposure rate, but there is a certain dispersion among them due to differences of sensitivity. In order to correct such a dispersion and to compensate for the sensitivity differences of the Geiger tubes, within those of the same type, the passage of the pulses from the Geiger tube through the pulse channel of the probe is periodically blocked for fixed and adjustable periods of time so that the probe delivers a fixed number of pulses per second for a given value of the exposure rate. The procedure is characterised by the possibility of calibrating and adjusting the probe independently of the measuring equipment.
The second channel of those mentioned is the current channel. It acts when the value of the mean current which passes through the Geiger tube exceeds a certain value. When this occurs, a current-to-pulse frequency conversion takes place, with a characteristic of proportionality between the current of the Geiger tube and the logarithm of the pulse frequency.
The pulses from this channel are mixed, or added in the Boolean sense, in a suitable device, to those from the pulse channel, thus obtaining an approximately linear characteristic between the exposure rate and the pulse frequency at the output of the mixing device.
When the exposure rate has very high values at which the Geiger tube is saturated, and does not produce electric pulses, the current channel continues supplying pules of a frequency greater than that corresponding to the upper limit of the measurement range; in such circumstances the indication of the measurement does not diminish, neither does it reach zero, thus avoiding the possibility of accidents due to false indication at high exposure rates.
Since the probe can be subjected to wide variations of temperature and since, on the other hand, there appears to be a certain degree of evidence that the useful life of a Geiger tube is shortened by rapid drops in temperature, the tube is housed in a suitable thermally insulated enclosure, the temperature of which is prevented from dropping from a pre-adjusted value.
In cases in which the probe may be subjected to severe conditions of impact or vibrations, steps are taken, as another form of non-limitative embodiment, to encapsulate the Geiger tube and the electronic circuit in a silicone elastomer or in another material having suitable physical properties.